1. Field of the Invention
This invention relates to a sighting apparatus using a laser beam, and more particularly to such apparatus which is capable of emitting a laser beam in any horizontal direction by remote control.
2. Description of the Prior Art
A sighting apparatus of this kind is already disclosed in the Japanese Patent Application Laid-open Nos. 215913/1986 and 215914/1986. This apparatus includes a rotatable table supported on a base, a laser oscillator and an optical system which guides the laser beams emitted from the laser oscillator in the horizontal direction whereby the table is rotated by remote control. Even a sole user can perform a survey using this apparatus, so that the use of this apparatus has spread.
As shown in FIG. 5, the optical system includes a collection lens 3 which collects the laser beams 2 emitted from the laser oscillator 1, an automatic correction mechanism 4 which corrects the laser beams passing through the collection lens 3 so as to emit horizontally, an objective 5 which images the laser beams from correction mechanism 4 onto an object as a reference point or line, and a focusing lens 6 which is disposed movably in the direction of the optical axis between the correction mechanism 4 and the objective 5.
For example, as shown in FIG. 6, the automatic correction mechanism 4 includes three mirrors 4, namely, two inclined mirrors 7a and 7b and a flat-plate mirror C. The two inclined mirrors 7a and 7b are fixed to the casing 8, and the flat-plate mirrors 7c is suspended by a plurality of (usually four) suspended threads 10 to a support plate 9 to which the casing 8 is fixed. Thus even if the casing or the table may be inclined, the flat-plate mirror 7c is translated so that the laser beams are emitted horizontally at all times. Thus even if the sighting apparatus may be inclined for some reason during remote control, the direction in which the laser beams are emitted is corrected due to the presence of the automatic correction mechanism 4 to maintain the reliability on the accuracy of the apparatus.
However, of course, in the mechanism 4, the range of movement of flat-plate mirror 7c is limited within a fixed range, so that when the mirror 7c is moved out of the effective operation range, the laser beams are not emitted horizontally and erroneous survey may be performed. Especially, in the sighting apparatus which uses remote control, the user performs a survey at a place remote from the aligning apparatus, so that he cannot ascertain the occurrence of the trouble in advance and the accuracy of survey itself is seriously influenced